Flexible drive couplings are transmission devices that connect between a driving and driven member, such as in a drive train, to provide misalignment accommodation, torque carrying capability and appropriate stiffness for vibration isolation. Couplings are used, for example, in a drive train between an engine and a unit to be rotated, such as a jet drive unit in a personal watercraft or a propeller in a boat. The coupling's torsional stiffness is designed to minimize torsional vibrations that may cause damage to the drive train components. Moreover, such couplings, as taught in U.S. Pat. No. 4,516,956 to Staiert, and U.S. Pat. No. 4,041,730 to Kress may include a torque overload feature where the bonded member slips inside the housing after a limit torque is exceeded. This may occur, for example, when the driven component becomes jammed or when it strikes another object.
Further Prior Art couplings are shown in FIGS. 1 and 2. These coupling connect between a flywheel and a output shaft in a personal watercraft. Each coupling 10 includes a driving member 11, a driven member 12, and a elastomer member 13 positioned between them. The elastomer member 13 is bonded to the driven member 12 and is received in an interference fit (precompressed) and unbonded relationship in a pocket 14 formed in the driving member 11. The FIG. 1 Prior Art coupling includes a low cocking stiffness of about 14,400 lbf.-in./radian (1,627 N-m/radian). This low stiffness prevents any parallel or cocking misalignment between the members 11, 12 from being converted into large radial forces which are then transmitted through engine mounts into the hull liner (frame), and finally to the operator of the personal watercraft. However, the FIG. 1 coupling includes a low radial stiffness, about 56,200 lbf./in. (9,835 N/mm). Any rotational unbalance present will be aggravated at higher rotational frequencies because the unbalance tends to move further outward from the central axis because of the low radial spring rate. Moreover, the concentricity between the driving and driven member can be poor when a low radial stiffness is provided, thereby possibly further aggravating any unbalance present.
To combat the low radial stiffness, a pivot bearing 15 was added in the FIG. 2 Prior Art coupling. This substantially increased the radial stiffness to approximately 219,000 lbf./in. (38,352 N/m), thereby improving any unbalance problem present. However, the positioning of the pivot bearing 15 is offset from the elastomer member 13, therefore, any parallel or cocking misalignment between the members 11, 12 causes the elastomer member to be loaded in radial compression. This results in a much higher cocking stiffness (approximately 426,700 lbf.-in./radian (48,217 N-m/radian)) than compared in the FIG. 1 coupling, and, therefore, resultantly higher loads generated should any cocking or parallel misalignment be present. Moreover, because of the high cocking stiffness it may be necessary to shim various driveline components to minimized such cocking or parallel misalignment, thus increasing manufacturing costs.
Although, in general, these prior art couplings have adequate performance and/or durability, they each exhibit certain performance limitations. For example, the FIG. 1 embodiment exhibits low radial stiffness thereby, in some installations, this can lead to unwanted radial vibrations in the drive train due to rotational unbalances in, and concentricity between, the members 11, 12. In an effort to provide increased radial stiffness, a pivot bearing 15 was added in the FIG. 2 embodiment. However, this pivot bearing 15 limits the degree of cocking misalignment that is achievable by the coupling as well as substantially increases the cocking stiffness thereof.
Accordingly, there has been a long felt, and unmet need for a coupling capable of transmitting torques, which exhibits both increased radial stiffness as well as low cocking stiffness.